1. Field of the Invention
The present invention relates to an antenna duplexer, and more specifically to an antenna duplexer including a transmission filter and a receiving filter. The present invention also relates to an RF module and a communication apparatus using such an antenna duplexer.
2. Description of the Background Art
Recently, along with the development of mobile communication, there have been demands for devices used for mobile communication which provide higher performance with smaller sizes. Antenna duplexers, as well as other devices, are being more and more reduced in size by using surface acoustic wave filters (SAW filters) or filters using film bulk acoustic resonators (FBARs) Semiconductor devices such as mixers and low noise amplifiers are being altered to have a balanced structure for the purpose of improving the noise characteristic against inter-device crosstalk and the like. Antenna duplexers to be used in connection with such semiconductor devices need to have a balanced structure.
FIG. 15 shows a structure of an antenna duplexer described in Japanese Laid-Open Patent Publication No. 2001-24476. As shown in FIG. 15, one conventional type of antenna duplexer uses a band pass filter having a ladder type circuit including FBARs. This conventional antenna duplexer (represented by reference numeral 10) includes an antenna terminal 11, a transmission terminal 12, a receiving terminal 13, a transmission filter 14, a phase shifter 15, and a receiving filter 16. The antenna terminal 11 is connected to the transmission terminal 12 via the transmission filter 14 and is also connected to the receiving terminal 13 via the 90° phase shifter 15 and the receiving filter 16. The 90° phase shifter 15 and the receiving filter 16 are connected in series. The transmission filter 14 include FBARs 14a, 14b and 14c connected in series to form a series resonator, and FBARs 14d and 14e connected in parallel to the series resonator. The FBARs 14a, 14b and 14c and the FBARs 14d and 14e are connected to form a ladder type circuit. The receiving filter 16 include FBARs 16a, 16b and 16c connected in series to form a series resonator, and FBARs 16d, 16e, 16f and 16g connected in parallel to the series resonator. The FBARs 16a, 16b and 16c and the FBARs 16d, 16e, 16f and 16g are connected to form a ladder type circuit.
In the case of, for example, a PCS (Personal Communication System), the transmission filter 14 and the receiving filter 16 are configured such that the high frequency rejection band of the transmission filter 14 overlaps the passband of the receiving filter 16 and such that the low frequency rejection band of the receiving filter 16 overlaps the passband of the transmission filter 14.
FIG. 16 shows a structure of an antenna duplexer described in Japanese Laid-Open Patent Publication No. 2003-249842. As shown in FIG. 16, one conventional type of antenna duplexer uses a band pass filter having a ladder type circuit including SAW filters. This conventional antenna duplexer (represented by reference numeral 20) includes an antenna terminal 11, a transmission terminal 12, a receiving terminal 13, a transmission filter 21, a phase shifter 15, and a receiving filter 22. The antenna terminal 11 is connected to the transmission terminal 12 via the transmission filter 21 and is also connected to the receiving terminal 13 via the 90° phase shifter 15 and the receiving filter 22. The 90° phase shifter 15 and the receiving filter 22 are connected in series. The transmission filter 21 include SAW resonators 21a, 21b and 21c connected in series to form a series resonator, and SAW resonators 21d and 21e connected in parallel to the series resonator. The SAW resonators 21a, 21b and 21c and the SAW resonators 21d and 21e are connected to form a ladder type circuit. The receiving filter 22 includes SAW resonators 22a and 22b connected in parallel to the receiving terminal 13.
In the case of, for example, a PCS (Personal Communication System), the transmission filter 21 and the receiving filter 22 are configured such that the high frequency rejection band of the transmission filter 21 overlaps the passband of the receiving filter 22 and such that the low frequency rejection band of the receiving filter 22 overlaps the passband of the transmission filter 21.
As described above, the conventional antenna duplexers have a ladder circuit including FBARs or SAW resonators. However, the transmission terminal and the receiving terminal in the conventional antenna duplexers are of unbalanced type. Therefore, a semiconductor device or the like having a balanced type terminal cannot be directly connected to such an antenna duplexer. In addition, the conventional antenna duplexers, which include an unbalanced type terminal, have characteristics which are deteriorated by the influence of noise such as crosstalk and the like.